This invention relates to hydraulic braking systems of the brake-by-wire type for vehicles of the kind in which hydraulic fluid from a high pressure source is supplied to a wheel brake under the control of a proportional solenoid-operated valve which, in turn, is actuated in response to signals from a pressure transducer associated with a brake-applying pedal.
In known brake-by-wire systems of the kind set forth the high pressure source comprises an hydraulic accumulator which is adapted to be charged by operation of an hydraulic pump. The pressure in the accumulator is determined by a pressure sensor working in conjunction with an isolating valve which is operative to isolate the pump from the accumulator when the pressure in the accumulator attains a given value. This valve effectively isolates the accumulator from the proportional operated solenoid valves so that the accumulator is not discharged due to leakage through the proportional operated solenoid valves when left to stand for a long period.
In one known system of the kind set forth the pressure sensor is located in the first line between the pump and proportional valve, and the isolating valve is located in a second line between the accumulator and a connection in the first line between the pressure sensor and the proportional valve. The relative positions of the pressure sensor and the isolating valve, under certain circumstances, may lead to concerns about reliability, power consumption, safety and packaging of the system. This is because the isolating valve must be opened in order to measure the accumulator pressure.
With regard to reliability the pressure sensor, isolating valve and proportional valve may all be subjected to large-magnitude pressure transients if pressure in a down stream high-pressure supply gallery decays before the isolating valve re-opens. This would occur if pressure in the first line decays over time due to leakage and then the isolating valve is suddenly opened introducing the accumulator pressure to the first line.
Excessive power consumption, leakage and consequent wear and energy consumption (recharging accumulator) will occur if the isolating valve is held open between successive brake applications in order to reduce accumulator-sensor fatigue due to supply gallery decays before the isolating valve re-opens. In addition excess heat dissipation from the solenoid of the isolating valve may occur. The isolating valve must incorporate a spring capable of opposing maximum accumulator pressure to keep the accumulator charged when the supply line pressure has decayed through leakage. Thus a high solenoid force must be maintained after opening to oppose this high spring force.
Since the accumulator pressure is monitored only when the isolating valve is open, periodic opening takes place for monitoring purposes but such movements may exacerbate the reliability problems discussed above. As a result since accumulator faults may not be detected immediately the driver may be unaware of faults when braking is required.